The vinyl compound is a kind of important chemical raw material, the polymeric characteristic of its unsaturated double bond (active vinyl) makes the vinyl compound to be used as a polymerization monomer to prepare various polymers.
The vinyl compound obtained by chemical synthesis or separation process usually contains various impurities. It is necessary to remove the impurities from the vinyl compound by rectification since they will negatively effect the sequent polymerization process.
However, the vinyl compound tends to proceed frequently undesired spontaneous polymerization under the high temperature and the high pressure during its rectification. Taking the methyl (meth)acrylate, for example, because of the α,β-unsaturated double bond (active vinyl), it has a tendency to proceed free radical polymerization, especially tends to polymerize spontaneously (i.e., spontaneous polymerization) under the effect of heat or light. Polymers formed by the spontaneous polymerization during rectification will deposit on the surface of the evaporator (because of its high temperature, the probability of forming the polymer is increased), thereby undesirably deteriorating heat transfer and clogging internals of the rectification column, causing unwanted pressure loss, and finally resulting in the stoppage of the rectification column. The interruption of the operation is unavoidable during rectifying the vinyl compound, therefore, to laboriously remove polymers formed.
In order to ensure a stable and continuous operation of the vinyl compound-rectification column for a long period of time, the prior art has developed methods for preventing clogging, such as a method of blowing a gas having a polymerization inhibiting action, or adding a sort or sorts of known polymerization inhibitors thereinto. Conventional polymerization inhibitors (hereinafter referred to as “inhibitor”) include N-oxyl compounds such as tert-butyl nitroxide, 2,2,6,6-tetramethyl-4-hydroxypiperidyl-1-oxyl, 2,2,6,6-tetramethylpiperidyl-1-oxyl, 2,2,5,5-tetramethyl-1-oxyl-pyrrolidine, 2,2,6,6-tetramethylpiperidino-oxyl and 4-hydroxy-2,2,6,6-tetramethylpiperidino-oxyl; phenol compounds such as hydroquinone, methoquinone, pyrogallol, catechol and resorcin; phenothiazine compounds such as phenothiazine, 3,7-dioctyl phenothiazine and bis-(α-dimethylbenzyl)phenothiazine; and copper compounds such as cupric chloride, copper acetate, copper carbonate, copper acrylate, copper dimethyldithiocarbamate, copper diethyldithiocarbamate and copper dibutyldithiocarbamate; as well as manganese salt, etc.
Polymerization is a complex chemical reaction procedure, which usually comprises the following three steps: chain initiation, chain growth, and implosion. Firstly, the monomer produces living free radicals under the effect of light, heat, or initiator. When one of the free radicals collides with another monomer molecule, a reaction occurs, forming new free radicals that are multiples of the monomer molecules to proceed chain growth process. When the spontaneous polymerization of the monomer proceeds to a certain degree, the process stream will become viscous, the popcorn type polymers tend to be formed easily and the phenomenon of implosion is likely to be observed. The formation mechanism of popcorn type polymer is comparatively complex, any of the tiny particles in popcorn type in the raw materials, the rusts or the burrs remained on the surface of the process pipe may initiate the formation and quick propagation of the popcorn type polymer. The key for preventing the spontaneous polymerization is to control the chain propagation, which can be attained by the addition of various high efficient inhibitors. The oxygen in the air itself is not only an efficient inhibitor, but also can make the other inhibitors performing their function of preventing polymerization to the greatest extent as well.
However, even if the above methods have been used, the phenomenon of spontaneous polymerization during rectifying the vinyl compound, such as methyl (meth)acrylate, will still frequently occur, although the spontaneous polymerization has somewhat been inhibited as compared to the rectifying process without the above methods. As a result, it is difficult to ensure a stable and continuous operation of the vinyl compound-rectification column for a long period of time, needing to remove the polymer formed by the spontaneous polymerization from the rectification column periodically.
Accordingly, it is still necessity to develop a method for rectifying vinyl compound, which method is capable of preventing the vinyl compound from spontaneous polymerization during its rectification process.